On a conventional optical disk, generally, information (hereinafter referred to as address information) indicating the addresses of track and sectors is prerecorded in the forms of bumps and pits at manufacture (the bumps and pits providing the address information, hereinafter referred to as address marks). The allocation of data thus prerecorded on an optical disk is called pre-format or hard-format. Recording, reproduction and erasing of desired information are carried out while optically detecting the address information with an optical head of an optical disk drive and identifying a desired track address and sector address.
Regarding the address marks, as illustrated in FIG. 27, marks 2 may be intermittently formed on the track provided between preformed grooves 1 on the optical disk. Also as illustrated in FIG. 29, marks 4 may be formed by partially interrupting grooves 3.
A desired location on the optical disk is accessed by access process including track seeking and sector searching operations. In the track seeking operation, the optical head is moved in a radial direction of the optical disk towards a desired location. In the following sector searching operation, the optical head waits for a specified sector to come to a projection position of a light spot on the optical disk by rotation.
On the optical disk disclosed in the Japanese Publication for Unexamined Patent No. 64-60823/1989, address marks are formed separately from the preformed grooves. In this configuration, during track seeking, the number of tracks crossed by a light spot is counted in accordance with a crossing signal which is detected when the light spot crosses the preformed groove. Then, the optical head is moved in a radial direction of the optical disk towards a location on the optical disk, calculated according to the numerated data.
However, as illustrated in FIG. 27, when the marks 2 are written between the preformed grooves 1, sometimes the light spot of a laser light beam 5 emitted by the optical head crosses not only the preformed groove 1 but also the mark 2 as shown by arrow .alpha. during track seeking. This causes a signal that is generated as the light spot 5 crosses the mark 2 to be added to a track crossing signal EE (see FIG. 28). As a result, three pulses are generated as shown by the dotted line instead of two expected pulses that correspond to two preformed grooves 1 adjoining the mark 2. Moreover, when the intervals between the preformed grooves 1 and the mark 2 are small, only one pulse as illustrated by the solid line may be generated with respect to the two preformed grooves 1 and mark 2. Such incorrect numbers of pulses cause errors in counting of the track crossing number.
Meanwhile, as illustrated in FIG. 29, when the marks 4 are formed by the intermittent preformed grooves 3, sometimes the light spot 5 crosses a blank area containing no mark as shown by arrow .alpha.. This causes a problem that a track crossing signal EF (see FIG. 30) is not detected even when the light spot 5 crosses the track.
With the above-mentioned method, since the address information of track and sectors are prerecorded as address marks in the form of bumps and pits, the format is inflexible. Therefore, the user can not freely change the format of the optical disk after manufacturing, and consequently the use of the optical disk is limited.
On the contrary, information such as address information is magnetically written on a conventional floppy disk after the floppy disk is manufactured. This method is hereinafter called soft-formatting. In other words, as illustrated in FIG. 31 and FIG. 32, a floppy disk 6 is provided with an index hole 7 for determining the start or reference position of soft-formatting. When the index hole 7 is detected by a photo-interrupter 8 (a detector incorporating a light emitting element and light receiving element) of a floppy disk drive, an index hole detection signal U is sent to an information recording and reproducing circuit 10. According to the detection signal U, the information recording and reproducing circuit 10 determines the start or reference position of soft-formatting. Then, a recording signal V is sent to a magnetic head 11, whereby address information is recorded on the floppy disk 6.
When soft-formatting used for the floppy disk 6 is adopted by an optical disk, the following problems are presented. Specifically, the recording density of the optical disk is substantially twenty times to some hundred times that of the floppy disk 6. Therefore, in determining the recording location on the disk, the optical disk requires substantially twenty times to some hundred times the accuracy of that required by the floppy disk. Thus, even when the optical disk is provided with the above-mentioned index hole 7, it is impossible to ensure sufficiently high-accuracy in determining the recording location.